Thousands more troops could be sent to Afghanistan within months as the UK government considers a change in strategy following the death of 15 British soldiers in 11 days.

Last month, military chiefs were dismayed to learn their requests for 2,000 more troops had been turned down because of a Treasury spending cap, but there are signs of a change of attitude within government.

The Ministry of Defence is now considering a troop surge in the face of mounting criticism that British forces in Afghanistan are understaffed and ill-equipped for battle with the Taliban, after the death toll of British soldiers in the country surpassed that in Iraq.

An MoD source confirmed to the Observer that ministers were re-examining troop numbers in Helmand while the prime minister, Gordon Brown, stressed in an address to British forces today that troop levels would be kept under review "in the light of circumstances".

After a week in which the government has come under pressure from the Liberal Democrat leader Nick Clegg, who claimed the lives of British soldiers were being "thrown away" because of a lack of political will, the prime minister insisted there was a "clear strategy".

"This is a patriotic duty," said Brown, speaking before news emerged that four US troops died yesterday in Afghanistan.

"Of course, people want to know if the action we are taking is the right action. It comes back to terrorism on the streets of Britain. If we were to allow the Taliban to be back in power in Afghanistan and al-Qaida then to have the freedom of manoeuvre it had before 2001, then we would be less safe as a country.

"There is a line of terror – what you might call a chain of terror – that links what's happening in Afghanistan and Pakistan to the streets of Britain."

Brown said the whole of the country would want to acknowledge the recent sacrifices by British troops, which included eight deaths in 24 hours. Five of the soldiers were killed by two improvised explosive devices while on foot patrol. After the first device exploded, the soldiers retreated to a place where the Taliban – predicting their reaction – had planted a second, bigger device. The attack shows the Taliban is adopting more sophisticated tactics, previously used against British troops in Northern Ireland.

Despite the casualties inflicted by the Taliban, and the apparent sophistication of the insurgents' tactics, Brown maintained that considerable progress was being made in the Operation Panther's Claw offensive to drive the Taliban from central Helmand province.

"I think the operation we are engaged with is showing signs of success. Our troops are making progress as they attempt to make the area safe," he said.

"The reports that I have show that despite the loss of life – and it is tragic and it is very, very sad indeed – our forces are doing a magnificent job in moving forward."

But the government strategy in Afghanistan was questioned today by the former Conservative defence secretary Lord King of Bridgwater, who said the British army was fighting a "critical offensive" with a serious shortage of helicopters and soldiers.

"Some of the equipment is getting a tremendous bashing out there... but where there really is a shortage, a really critical shortage, is helicopters," said Lord King, who was at the Ministry of Defence in John Major's government during the first Gulf war, on BBC1's Andrew Marr Show. King, also a former Northern Ireland secretary, said that when the roads became too dangerous in Northern Ireland, troops switched to helicopters but the lack of helicopters in Afghanistan restricted such an option.

The prime minister said today that the government had spent more than £1bn acquiring more than 1,000 new armoured vehicles since 2008, including 280 Mastiffs. They would be followed later in the year by new Ridgebacks.

At the same time, he said commanders in Afghanistan had almost twice as much helicopter capability as they did two years ago.

The defence secretary, Bob Ainsworth, said Britain had "just about the most capable armed forces in Afghanistan".

"They will tell you that they have seen a massive improvement in personal kit and equipment... and vehicles," Ainsworth told BBC News today.

• A British soldier killed in southern Afghanistan on Friday was named as Corporal Lee Scott. The 26-year-old, from the 2nd Royal Tank Regiment, was married with two young children. He died in an explosion near Nad-e-Ali in Helmand province.